Modern commercial reproduction systems such as printers incorporate exposure devices, such as raster output line scanning (ROS) laser beams or a linear transverse array of light emitting diodes, to sequentially, line by line, project images onto a moving photoreceptor. It is known that undesirable image exposure strobing and the like can result from mechanical vibrations of the components comprising the exposure device or the photoreceptor or from speed variations in the photoreceptor. The undesirable effects may include unscanned interstices between adjacent scan lines or overlapped scan lines. Strobing effects can occur when the major spectral content of the mechanical vibrations is in the 1 Hz to 500 Hz range. However, with typical photoreceptor process speeds of 100-400 mm/sec, frequencies less than 10 Hz do not normally produce visible strobing effects on the copies, due to the restricted sensitivity of the human eye thereto. Thus, the range of mechanical vibrations producing undesirable strobing effects is effectively between 10 Hz and 500 Hz. Of course, lower frequency photoreceptor speed fluctuations produce other image distortions.
Such strobing effects have been a long-standing problem in xerographic laser printers. Similar distortion or misregistration problems also occur in color xerography where color images must be precisely aligned or positioned on top of each other. Many ideas have been proposed to compensate for the distortions caused by the mechanical vibrations of the photoreceptor and the exposure device, or for variations in the speed of the photoreceptor.
For example, it is known to servo-control the speed of the photoreceptor. For example, U.S. Pat. Nos. 4,963,899 discloses a servo-controlled drive to control the speed and position of the photoreceptor so that image frames are properly registered on the photoreceptor. U.S. Pat. No. 4,837,636 discloses a motion sensor for sensing the relative position and velocity of the photoreceptor. Regarding other servo control systems, see, e.g., U.S. Pat. No. 4,332,461 which discloses a servo motor control for driving document scanning carriages in a continuously variable reduction copier.
Other ideas proposed to remedy strobing effects include controlling the exposure device. U.S. Pat. No. 4,427,275 discloses a system for non-impact printers such as those using LED's to eliminate discontinuity created because of the movement of the imaging surface relative to the LED display, through the adjustment of the enabling sequence of the LED's. U.S. Pat. No. 4,801,978 discloses an electronic printer which utilizes an encoder positioned on the photoreceptor to determine velocity and vibrational changes in the photoreceptor and to create an electrical signal to control the on-off time and intensity of the LED's to compensate for those changes.
In ROS type machines it has been proposed to control the speed of the polygonal scanning motor or the speed of the photoreceptor. U.S. Pat. No. 4,349,847 discloses a ROS having a laser beam and a rotating polygon scanning the beam across a charged photoconductive belt. The belt has timing marks which are detected by a light source and photosensor. The output from the photosensor corresponds to the belt velocity and this signal is used to control the angular velocity of the polygon. U.S. Pat. No. 4,975,626 also discloses a circuit for controlling the speed of the polygon motor.
Still other ideas proposed include controlling the tilt or angle of the beam scanning the images on the photoreceptor. U.S. Pat. No. 4,682,842 discloses a polygon scanner system for correcting wobble by double reflection of a light beam from a pair of fixed mirrors. U.S. Pat. No. 4,560,244 discloses a wobble-plate apparatus for redirecting an incident beam on target. U.S. Pat. No. 4,595,295 discloses a dual wobble-plate apparatus for refracting light in both the X and Y directions. U.S. Pat. Nos. 4,600,837 and 4,661,699 disclose systems for controlling the incident scanning beam path. U.S. Pat. No. 4,561,023 discloses a micro-deflector which deflects light impinging thereagainst, on the application of a bending potential, and which includes a dampening system to speed up restoration of the deflector to an undeflected quiescent position following removal of the bending potential.
A suggestion has been made for limiting strobing effects by controlling the tilt or angle of the laser beam scanning the images on the photoreceptor and the speed of the photoreceptor. Of particular interest is the above-cited allowed application Ser. No. 07/524,895, by Joseph Fantuzzo, now entitled "Raster Scanner Including Scanning Beam Tilt Correction" and to a corresponding article entitled, "Raster Output Scanner", published November/December 1990 in the Xerox Disclosure Journal (XDJ), Vol. 15, No. 6, pages 455-461, both of which disclose a ROS having a laser beam and a rotating polygon for scanning the beam across a charged photoreceptor, with "means for detecting the position of the beam relative to the imaging member (the photoreceptor) and generating a positive signal thereof, . . . [m]eans, responsive to the position signal, [for] control[ling] the velocity of the imaging member relative to the beam, . . . [m]eans responsive to the position signal, [for] adjust[ing] the tilt of the beam relative to the imaging member". [XDJ pg. 458, lines 30-35]. It is important to note that the means that said Fantuzzo discloses for detecting the position of the beam relative to the photoreceptor member is timing marks on the photoreceptor and sensors which detect speed variations and mechanical vibrations of the photoreceptor. Also, it is important to note in Fantuzzo that the position signal generated therefrom is used asynchronously, first to control the speed of the photoreceptor and then to control the tilt of the beam.
Prior suggested control arrangements such as these noted above are subject to various problems and shortcomings. For example, the above-mentioned proposed systems apparently do not compensate for relative vibrational motion between the belt module and the exposure device in the process direction. Such relative vibration could be induced by, for example, paper hitting a printer component, an operator closing a paper drawer, or a stitcher actuating or the like. The photoreceptor velocity encoder and control mechanisms discussed above only take account of speed and vibrational distortions in the photoreceptor per se.
Importantly, because of the high inertia of the moving photoreceptor elements, a servo or other similar speed control mechanism capable of responding to high frequency drive signals of over 300 Hz with the requisite very high accelerations and decelerations would be very expensive.
The disclosed apparatus may be readily operated and controlled in a conventional manner with conventional control systems. Some additional examples of various prior art copiers with control systems therefor are disclosed in U.S. Pat. Nos. 4,054,380; 4,062,061; 4,076,408; 4,078,787; 4,099,860; 4,125,325; 4,132,401; 4,144,550; 4,158,500; 4,176,945; 4,179,215; 4,299,101; 4,278,344; 4,284,270, and 4,475,156. It is well known in general and preferable to program and execute such control functions and logic with conventional software instructions for conventional microprocessors. This is taught by the above and other patents and various commercial copiers. Such software may of course vary depending on the particular function and the particular software system and the particular microprocessor or microcomputer system being utilized, but will be available to or readily programmable by those skilled in the applicable arts without undue experimentation from either verbal functional descriptions, such as those provided herein, and prior knowledge of those functions which are conventional, together with general knowledge in the software and computer arts. Controls may alternatively be provided utilizing various other known or suitable hard-wired logic or switching systems.
As shown in the above-cited art, the control of exemplary servo drivers and optical beam deflectors, and, for example, document and copy sheet feeding systems in copiers, may be accomplished by conventionally actuating them by signals from the copier controller directly or indirectly in response to simple programmed commands and from selected actuation or non-actuation of positional velocity, or vibrational sensors, etc.. The resultant controller signals may conventionally actuate various conventional electrically or mechanically-controlled sheet or optical beam deflectors, motors or clutches in the copier in the selected steps or sequences are programmed. Such microprocessor control circuitry and sensors may be utilized for determining the speed of the photoreceptor and mechanical vibrations of the photoreceptors, etc. and thereby controlling the operation of the photoreceptor servo motor, optical beam deflectors, etc., as further described herein.
All references cited above in this specification, and their references, are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features, and/or technical background.
It is a feature or advantage of this invention to overcome various of the above-noted and other problems, and to provide a reproduction apparatus and method with reduced image quality distortions due to mechanical vibrations of the photoreceptor or the exposure device or variations in the speed of the photoreceptor, at reasonable cost. Unscanned interstices between adjacent scan lines or overlapped scan lines on the photoreceptor may thus be avoided.
An additional specific disclosed feature or advantage is to provide separation, into high and low frequency signals, of signals corresponding to mechanical vibrations of the photoreceptor or the exposure device or variations in the speed of the photoreceptor, for separate image distortion correction.
Another disclosed feature or advantage is to provide an optical system for compensating for high frequency distortions due to mechanical vibrations of or between the photoreceptor and the exposure device.
A further disclosed feature or advantage is to provide, in coordination, a low cost servo to drive the photoreceptor and to respond to low frequency distortions due to mechanical vibrations of the photoreceptor or the exposure device or variations in the speed of the photoreceptor.
A specific feature of the disclosed embodiment is correct image distortion in a reproduction machine due to vibrational and speed distortions in or between a moving photoconductive member and an electro-optical imaging device optically line scanning the photoconductive member as the photoconductive member moves past the imaging device, comprising the steps of: encoding, in electrical signals, the speed of the photoconductive member and also the relative vibrational motion between the photoconductive member and the imaging device; feeding said electrical signals to a signal separator; separating said electrical signals into lower frequency signals and higher frequency signals with said signal separator; feeding said lower frequency signals to a servo drive means to control the speed of the photoconductive member; and feeding said higher frequency signals to electrically controlled means (preferably, an electro-optical beam shifting transducer) for adjusting the position that said imaging device optically line scans.
The foregoing and other features and advantages of the present invention may be accomplished by the disclosed embodiment of an image distortion correction apparatus or method in a reproduction machine which corrects image distortion due to vibrational or speed distortions in or between a moving photoreceptor and an imaging device projecting scan lines on the photoreceptor as it moves past the imaging device. This exemplary image distortion correction apparatus preferably includes a ROS-mounted encoder for encoding the speed of the photoreceptor and also the relative vibrational motion between the photoreceptor and the imaging device, and for providing corresponding electrical signals therefrom, a signal separating for separating the electric signals into low frequency signals and high frequency signals, a servo motor to drive the photoreceptor and to compensate for any low frequency image distortions, and an optical pivoter to adjust the scan line projected onto the photoreceptor in order to compensate for any high frequency image distortions. The servo motor is driven by the low frequency signals and the optical pivoter is driven by the high frequency signals.
It should be noted that the subject image distortion correction apparatus is not limited to conventional xerographic printers and that it could easily be incorporated into a color printer or copier to more precisely register a second color latent image to or on top of a first color image previously put onto the photoreceptor.